1. Field of the Invention
The present invention relates to an information recording medium utilized for an optical recording/reproducing apparatus, which reads out information recorded on the information recording medium by making the information recording medium relatively move, particularly, relates to an information recording medium comprising a read only area of enabling to read out information and a recording/reproducing area of enabling to record and reproduce information.
2. Description of the Related Art
Currently various types of information recording mediums such as in a disk shape, in a card shape, and in a tape shape are utilized as an optical information recording medium from which information is read out by making the information recording medium relatively move. Such information recording mediums are divided into two types in consideration of mechanism of recording or reproducing: one is a read only type and the other is a recording/reproducing type that is, a recordable type and an overwritable type. A read only type information recording medium is shipped out into a market with being prerecorded with information such as a CD audio disk typically, and is reproduced by a user. On the other hand, a recordable or overwritable type information recording medium is shipped out into a market without any recorded information, and a user records information on it and reproduces it if necessary.
A problem arises along with a trend toward multimedia causing to increase a chance such that copyright is infringed by using an electronic means. Accordingly, in order to prevent such the problem, an idea such as embedding a specific code, which can not be rewritten by a user, is necessary to be realized even in a recordable or overwritable type information recording medium, which can be recorded freely by a user.
As mentioned above, various kinds of information recording mediums, which are provided with two areas composed of a read only area and a recording/reproducing area, have been introduced. For example, there is existed an information recording medium provided with a recording/reproducing area only in a predetermined angle. Further, another information recording medium having a recording/reproducing area, which is provided with a read only area allocated in each one half track, is introduced.
Furthermore, an information recording medium provided with both a read only area and a recording/reproducing area, which are allocated in different positions from each other, is introduced. FIG. 2 is a cross sectional view of such the information recording medium 301. In FIG. 2, the information recording medium 301 comprises a substrate 10, a recording layer 13, and a resin layer 14. A microscopic pattern is formed on a surface of the substrate 10 at a boundary area between the substrate 10 and the recording layer 13. Microscopic patterns constituting a read only area and a recording/reproducing area are engraved in areas corresponding to the read only and recording/reproducing areas respectively. Actually, microscopic patterns 11 corresponding to the read only area and other microscopic patterns 12 corresponding to the recording/reproducing area are engraved. These microscopic patterns 11 and 12 are different from each other in depth. A depth “d1” of the microscopic patterns 11 is λ/4n, on the other hand, a depth “d2” of the microscopic patterns 12 is λ/8n, wherein λ is a reproduction wavelength of a laser beam and “n” is a refractive index of the substrate 10 at the reproduction wavelength λ. Specifying the depth of the microscopic patterns 11 to λ/4n is caused by a phase depth in which a signal output from the read only area becomes maximum. Further, specifying the depth of the microscopic patterns 12 to λ/8n is caused by that a push-pull signal related to tracking of the information recording medium becomes a maximum output.
As mentioned above, allocating two areas in different positions reduces load of a recording/reproducing apparatus due to a simplified layout. Accordingly, there is much merit.
However, there is existed a following problem of the prior art mentioned above. A defect has occurred when an information recording medium formed with microscopic patterns shown in FIG. 2 has been loaded in various recording/reproducing apparatuses, which are currently available in a market, and operated. A mode of the defect is out of tracking while reproducing the information recording medium with continuously traversing from a recording/reproducing area to a read only area. Further, an operation traversing reversely from the read only area to the recording/reproducing area is also defective. According to our careful investigation of the problem, it is found that a recording/reproducing apparatus employs a push-pull method for tracking so as to record information in a groove and the push-pull method does not match with the information recording medium 301. In other words, firstly, the problem is caused by that forming the read only area in the depth of d1=λ/4n results a push-pull signal output in zero. In a case that the push-pull signal output is zero, a track can not be detected. Therefore, the reproduction stops at the track. Further, as a second problem, output difference between a push-pull signal output of the read only area and a push-pull signal output of the recording/reproducing area is large, so that a servo system of the recording/reproducing apparatus can not follow properly. Accordingly, it is required to solve such the problem as out of tracking when an information recording medium having two areas is actually recorded and reproduced.